1. Field of the Invention
The present invention relates to an optical duo-binary transmission apparatus, and more particularly to a parallel processing, duo-binary encoder and an optical duo-binary transmission apparatus using such an encoder.
2. Description of the Related Art
In general, a conventional Dense Wavelength Division Multiplexing (hereinafter, referred to as a DWDM) optical transmission system can transmit an optical signal having multiple channels with different wavelengths through a single optical fiber. The DWDM system can transmit an optical signal regardless of transmission speed. Because of these features, such DWDM systems are widely used in ultra-high speed Internet networks. Conventional systems are known that can transmit more than a hundred channels through a single optical fiber using the DWDM technology. In addition, research is being conducted to develop a system that can transmit more than two hundred channels of 40 Gbps through a single optical fiber simultaneously, thus having a transmission speed of more than 10 Tbps.
However, the enlargement of transmission capacity is restricted due to severe interference and distortion between channels. This is because the channel distance is less than 50 GHz when the light intensity is modulated using the conventional non-return-to-zero (NRZ) method, which is due to a rapid increase of data traffic and a request for high-speed transmission of data of more than 40 Gbps. Transmission distance is restricted in high-speed transmission of more than 10 Gbps since a direct current (DC) frequency component of a conventional binary NRZ transmission signal and a high frequency component spread during modulation cause non-linearity and dispersion when the binary NRZ transmission signal propagates in an optical fiber medium.
Optical duo-binary technology may be an optical transmission technology capable of overcoming restrictions in transmission distance due to chromatic dispersion. A main advantage of the duo-binary transmission is that the transmission spectrum is reduced in comparison to the general binary transmission. In a dispersion restriction system, the transmission distance is in inverse proportion to the square of the transmission spectrum bandwidth. This means that, when the transmission spectrum is reduced by {fraction (1/2,)} the transmission distance increases four times. Since the carrier frequency is suppressed in a duo-binary transmission spectrum, it is possible to relax the restriction of the optical power output caused by Brillouin scattering excited in the optical fiber.
FIG. 1 is a block diagram showing the construction of a conventional optical duo-binary transmission apparatus 100. The conventional optical duo-binary transmission apparatus 100 includes a multiplexer 101, a precoder 102, a low pass filter 103, a modulator driving amplifier 104, a laser source 105 for outputting a carrier, and a Mach-Zehnder interferometer type optical intensity modulator 106. The multiplexer 101 multiplexes data input signals of N number of channels and outputs the multiplexed signal. The precoder 102 then encodes the multiplexed signal. The low pass filter 103 converts a 2-level NRZ electrical signal output from the precoder 102 into a 3-level electrical signal and reduces the bandwidth of the signal. The modulator driving amplifier 104 amplifies the 3-level electrical signal to output an optical modulator driving signal.
Hereinafter, an operation of the conventional optical duo-binary transmission apparatus having the above-mentioned construction will be described.
Referring again to FIG. 1, the input signals of N number of channels are multiplexed by the multiplexer 101, and the multiplexed signal is then encoded by the precoder 102. The 2-level binary signal output from the precoder 102 is input to the low pass filter 103, and the low pass filter 103 has a bandwidth corresponding to about ¼ of a clock frequency of the 2-level binary signal. This excessive limitation to the bandwidth causes interference between codes, which thus changes the 2-level binary signal to a 3-level duo-binary signal. The 3-level duo-binary signal is then amplified by the modulator driving amplifier 104 and used as a driving signal of the Mach-Zehnder interferometer type optical intensity modulator 106. The carrier output from the laser source 105 is subjected to phase and optical intensity modulation according to the driving signal of the Mach-Zehnder interferometer type optical intensity modulator 106 and is then output as a 2-level optical duo-binary signal.
FIG. 2 is a view showing a pattern and a phase shift of an output optical signal when a signal having a data sequence of 11011000100110011101 is transmitted by means of the optical duo-binary transmission apparatus in FIG. 1. In FIG. 2, whenever the data input signal becomes ‘0’, the phase of the data input signal is shifted by π.
However, according to the prior art, in generating the 3-level data signal by the electric low pass filter, transmission characteristics deteriorate in a manner that depends on the pattern of an input signal.
Further, according to the prior art, since n number of input optical signals are multiplexed through a multiplexer, and the multiplexed data are then encoded by a precoder, data transmission speed increases n times in comparison with the transmission speed before multiplexing. The means that a high speed precoder that corresponds to the data transmission speed is required. However, in the case of the conventional precoder, it has a structure including an exclusive OR (hereinafter, referred to as an XOR) gate and a time delay unit for delaying an output signal of the XOR gate by 1 data bit and feed backing the delayed signal. Therefore, in the case of a high speed data signal, it is difficult to realize a high speed precoder due to time delay and limitation in the operational speed of the XOR gate.
In addition, such prior art systems have a characteristic in which a phase shift occurs at each ‘0’. It is noted that when the number of consecutive ‘0’s is even, the phase shift does not occur between data of the consecutive ‘0’s and at least one ‘1’ adjacent to the consecutive ‘0’s.
FIG. 3 is a block diagram showing the construction of another conventional optical duo-binary transmission apparatus. FIG. 4 is a view showing output signals at points {circle around (1)}, {circle around (2)}, {circle around (3)}, {circle around (4)}, and {circle around (5)} when the data sequence of 11011000100110011101 is transmitted by means of the optical duo-binary transmission apparatus shown in FIG. 3
In FIG. 3, the conventional optical duo-binary transmission apparatus 200 includes a multiplexer 201, an encoder 202, a coupler or an adder 203, a modulator driving amplifier 204, a laser source 205 for outputting a carrier, and a Mach-Zehnder interferometer type optical intensity modulator 206. The multiplexer 201 multiplexes data input signals of N number of channels and outputs the multiplexed signal, and the encoder 202 encodes the multiplexed signal so that the multiplexed signal includes phase information. The coupler 203 converts the encoded signal into a 3-level electrical signal, and the modulator driving amplifier 204 amplifies the 3-level electrical signal and outputs an optical modulator driving signal.
According to the conventional optical duo-binary transmission apparatus 200, a low pass filter and a precoder are unnecessary. Instead, in order to enable the apparatus to have a phase shift which is main characteristic of an optical duo-binary signal, the encoder 202 outputs data {circle around (2)} having non-shifted phases and data {circle around (3)} requiring a phase shift. The output signals {circle around (2)} and {circle around (3)} of the encoder 202 a reconverted into a 3-level signal {circle around (4)} by the coupler 203, and the converted signal is passed through the optical intensity modulator 206 and is then output as an optical duo-binary signal {circle around (5)} with a phase shift.
Similar to the apparatus shown in FIG. 1, since the optical duo-binary transmission apparatus in FIG. 3 multiplexes the input signals of N number of channels and then encodes the multiplexed signal, the apparatus requires a high speed encoder. However, such a high speed precoder cannot be realized due to operation speed limitations in the electrical device constituting the encoder.